Invertebrate Anatomy OnLine
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This is one of many exercises available from Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology. Additional exercises can be accessed by clicking on the links in the left column. A glossary and chapters on supplies and laboratory techniques are also available. Terminology and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises.
Cnidaria P, Medusozoa, Hydrozoa C, Limnomedusae O, Olindiidae F (Fig 7-75, 7-74)
The cnidarian body consists of a central blind sac, the coelenteron (= gastrovascular cavity), enclosed by a body wall comprising two epithelia, the outer epidermis and the inner gastrodermis (Fig 7-1, 7-2). A gelatinous connective tissue layer, the mesoglea, lies between the two epithelia. The mouth opens at one end of the coelenteron and marks the oral end. The mouth is at the tip of a process, the manubrium that elevates it above the oral surface. The opposite pole is the aboral end. The imaginary line connecting the oral and aboral poles is the axis of symmetry around which the radial symmetry of the body is organized. The mouth is usually surrounded by one or more circles of tentacles.
The defining cnidarian feature is, of course, their possession of stinging cells, or cnidocytes (Fig 7-8). Characteristic of the epidermis, they are also sometimes found in the gastrodermis. Cnidocytes contain an explosive organelle, the cnida, which, upon proper stimulation, inverts and ejects a slender, often barbed and toxic thread in the direction of prey or predator (Fig 7-9). Three types of cnidae are found in cnidarians (Fig 7-10). Nematocysts (in nematocytes), spirocysts (in spirocytes), and ptychocysts (in ptychocytes). All toxic cnidae are nematocysts whereas spirocysts are sticky, and the everted tubules of ptychocysts are used for constructing feltlike tubes. Most cnidae are nematocysts and these are present in all three higher cnidarian taxa. Spirocysts and ptychocysts are found only in Anthozoa.
The basic body plan described above can be manifest as a swimming medusa or attached polyp. In some taxa only one generation is present whereas in others both are found. A life cycle featuring alternation of sexual, swimming medusae with benthic asexual polyps is typical of many cnidarians.
All cnidarians are carnivores feeding on live prey which they usually capture using tentacles armed with cnidocytes. Digestion occurs in the coelenteron which is typically equipped with ciliated canals for distribution of partly digested food. Cnidarians are ammonotelic and diffusion across the body and tentacle surface eliminated the ammonia from the body. Gas exchange is across the general body surface. The nervous system is a plexus of basiepithelial neurons serving sensory and motor systems (Fig 7-6). Most cnidarians are gonochoric. The life cycle typically includes a planula larva. Cnidarians are chiefly marine but the well-known Hydra is an exception.
Medusozoa comprises those cnidarians whose life cycle includes a medusa generation that alternates with a polyp generation (Fig 7-75B). Symmetry is radial and tetramerous. Nematocysts are the only type of cnidocyte present. Included taxa are Scyphozoa (jellyfishes) and Hydrozoa (hydroids, Hydra, Portuguese men of war, etc).
Hydrozoa is a diverse taxon of about 3000 species of mostly marine cnidarians. The life cycle usually includes both polyp and medusa generations (Fig 7-65A) but may be entirely polyp (Fig 7-65B) or entirely medusa (Fig 7-65C). Polyps typically are colonial and medusae usually solitary. Some form colonies of combinations of polyps and medusae. The few freshwater cnidarians, such as Hydra, Vallentinia, and Craspedacusta, are hydrozoans.
Hydrozoan polyps are usually small, about 1 mm in length, and colonial. Hydromedusae are also small, at least in comparison with scyphomedusae, and are usually less than 1 cm in diameter. Hydromedusae are further distinguished from scyphomedusae by possession of a velum, a circumferential shelf of tissue that encircles the subumbrellar concavity and functions as an adjustable diaphragm to create a pulse of water for swimming. Medusae are tetramerously symmetrical as are scyphomedusae and four radial canals and four tentacles are usually present.
Cnidocytes are found only in the epidermis and germ cells arise in the gastrodermis but may, or may not, migrate to the epidermis to form gonads and mature into gametes. Gonads may be on the manubrium or on the radial canals. Gametes, even if gastrodermal, are released directly to the surrounding water, never, in contrast with scyphomedusae, into the coelenteron.
Both polyps and medusa generations in the life cycle. Medusae are thick cup-shaped and large for hydromedusae. The reduced polyps, however, are tiny, typically solitary, and athecate. Polyps may asexually produce both planulae, which form additional polyps, and medusae, which enter the plankton. Statocysts, but not ocelli, are present and the gonads are on the radial canals.
1. Gonionemus Although polyps of the Limnomedusae are reduced and tiny, the medusae are larger than those of most hydrozoans and are easily studied. If living material is available it should by all means be used but in most laboratories preserved material will be provided instead (Fig 7-59B,C).
Gonionemus is a shallow-water marine species. The medusa is about 2 cm in diameter and has a thick bell. It is robust and can withstand gentle handling without being damaged. Place a medusa in a small, transparent, glass culture dish of water and study it with the dissecting microscope. It should be completely immersed in water (tapwater if preserved: seawater or magnesium chloride if alive). Start at low power, using higher power as needed. Use incident and transmitted light as needed. You will probably find transmitted light to be the most useful.
The body has the shape of a bowl but is referred to as the bell. Find the convex aboral surface (=exumbrella, (Fig 1) of the bell. The opposite oral surface, or subumbrella, is strongly concave. Orient the animal in the dish with the oral surface down so you are looking at the convex aboral surface. Gently spread the margins of the bell to their maximum diameter so that you can better see the internal structures.
The outer surface of the animal is covered by a thin, transparent, colorless, monolayered epidermis but this is not apparent in gross examination. The interior surfaces, i.e. those of the coelenteron, are lined by another thin, monolayered epithelium, the gastrodermis. It is more apparent and imparts a dark color to the walls of the coelenteron, which it lines. Between the single layer of cells of the epidermis and that of the gastrodermis is the thick, acellular, jellylike connective tissue known in cnidarians as mesoglea.
Look through the epidermis and mesoglea into the bell and observe the major structures. Most of them are lined by gastrodermis and are dark and easily seen.
The manubrium is a long tubular process that hangs into the subumbrellar space below the bell but from your present perspective all you see is its base. The base of the manubrium appears as a dark square in the center of the subumbrella. The light area at the base of the manubrium, is the stomach. This is the digestive region of the coelenteron and it is here that extracellular digestion occurs. The mouth, which you do not see from this angle, is at the opposite end of the manubrium. Radiating from the four corners of the stomach are four dark, narrowradial canals that extend from the stomach to the margin of the bell. They are dark and should be easy to see unless obscured by the four large frilly gonads associated with them. The radial canals join the circumferential ring canal that encircles the edge of the bell. The canals are the distributive portion of the coelenteron.
From the outer margin of the bell arise numerous hollow tentacles. The lumina of the tentacles are continuous with the lumen of the ring canal and are hence lined with gastrodermis and are part of the coelenteron. The tentacles are contracted in preserved specimens and would be longer in life.
Figure 1. The medusa of Gonionemus. hydrozoa6L.gif
Look at some tentacles at higher magnification. They are ringed with cnidocyte batteries. Near the distal end each tentacle bears an adhesive pad of secretory tissue. These sticky pads are used by the animal to attach temporarily to sea grasses and seaweeds. A dark, opaque, swollen tentacular bulb can be seen at the base of each tentacle. The bulb contains a diverticulum of the ring canal. In the bulbs, interstitial cells differentiate into cnidocytes that will be used to replace discharged cnidocytes on the tentacles.
Between the bases of successive tentacles are sense organs composed of a short tube open laterally to the sea and an interior statocyst. Try to demonstrate the opening with your microneedle. At the base of each is a swelling which houses the statocyst. The statocyst detects gravity and tells the animal which way is down.
Turn the animal over so the oral surface is up. It is concave with the pendulous manubrium in its center. Find the manubrium again and locate the mouth in the center of its free end. The mouth opening is surrounded by four frilly oral lobes, which are contracted in preserved material. Reflect the oral lobes to open the mouth and look inside the coelenteron. You are looking into the stomach, which you saw earlier.
Look at the corners of the stomach to find the openings of the four radial canals. The connections between the stomach and the canals will be easier to see from this side of the bell. Look again at the ribbonlike gonad on each radial canal. Follow a radial canal to the periphery and its connection with the ring canal. This search will take you beneath the velum, a thin circumferential shelf of tissue that encircles the subumbrellar concavity. It is a defining character of hydromedusae and is absent from scyphomedusae.
Bullough WS. 1958. Practical Invertebrate Anatomy (2 nd ed). MacMillan, London. 483p.
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
1 living or preserved Gonionemus medusa
1 dissecting microscope
1 culture dish